Optical scanning apparatus and technique for correcting optical characteristics in an image forming apparatus that employs an electrostatic recording method or an electrophotographic recording method

ABSTRACT

An optical scanning apparatus controls an output of a light source for forming an electrostatic latent image on an image carrier. The optical scanning apparatus includes a correction amount control unit configured to variably control a light quantity correction amount of the light source according to a scanning position on the image carrier during one scanning operation with a beam generated from the light source, an output signal level changing unit configured to change a level of an output signal from the correction amount control unit, and a light quantity control unit configured to control a light quantity of the light source according to the scanning position based on a signal from the output signal level changing unit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/949,580, filed Dec. 3, 2007, entitled “OPTICAL SCANNINGAPPARATUS AND TECHNIQUE FOR CORRECTING OPTICAL CHARACTERISTICS IN ANIMAGE FORMING APPARATUS THAT EMPLOYS AN ELECTROSTATIC RECORDING METHODOR AN ELECTROPHOTOGRAPHIC RECORDING METHOD”, the content of which isexpressly incorporated by reference herein in its entirety. Further, thepresent application claims priority from Japanese Patent Application No.2006-341126 filed Dec. 19, 2006, which is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a technique for correcting opticalcharacteristics in an image forming apparatus that employs anelectrostatic recording method or an electrophotographic recordingmethod.

2. Description of the Related Art

A conventional optical scanning apparatus is, for example, employed inan image forming apparatus of an electrophotographic type and configuredto hold a constant level of laser light quantity during one scanningoperation. To this end, a conventional control operation includesdetecting an output of the laser within a predetermined light detectionperiod (i.e., a beam detection (BD) period) being set during onescanning operation and holding the driving current for the laser at aconstant level during one scanning operation. This is generally referredto as auto power control (APC) processing. One scanning operation is asingle scanning operation of a laser beam in a longitudinal direction(i.e., an axial direction) of a photosensitive member (i.e., an imagecarrier).

However, when one scanning operation of the laser beam is performed on aphotosensitive member (i.e., an image carrier), the density of areproduced image varies depending on a laser position during thescanning operation. The unevenness of density is particular when imagesare compared between the center and the edge of the photosensitivemember.

In general, a luminous flux incident on a polygonal mirror of an opticalscanning apparatus has a Gauss distribution such that the lightintensity can be maximized in the vicinity of an optical axis of acollecting optical system. Therefore, a light reflection and deflectionregion changes from the vicinity of the optical axis toward the edgeaccording to an angle of field (i.e., a scanning angle capable ofassuring an effective writing width relative to a photosensitivemember).

The illuminance on a photosensitive member (i.e., a surface to bescanned) tends to decrease at the edge compared to the center. Namely,the illuminance decreases according to an increase in the image height(i.e., the position on a photosensitive member). The image height is “0”at the center of the photosensitive member and has a plus or minus valuewhen the position moves toward the edge. This phenomenon is hereinafterreferred to as “light quantity falloff at edges.”

In addition, due to an erroneous setup of a light source (such as alaser), the position where the intensity of an incident luminous flux ismaximized on a deflection surface of a polygonal mirror may deviate fromthe center of an effective luminous flux width (relative to a mainscanning direction of the deflection surface) toward an edge of theeffective luminous flux width. In this case, in addition to lightquantity falloff at edges, the illuminance on a scanned surface tends toincrease or decrease when the position moves from one image height toanother image height.

The illuminance along a scanning line on a photosensitive member (i.e.,a surface to be scanned) may cause unevenness. Therefore, a formed imagemay have unevenness of density.

As discussed in Japanese Patent Application Laid-Open No. 2006-069118,in order to solve the above-described problem (i.e., to correct lightquantity fall at edges), a conventional system divides one scanningperiod into a plurality of blocks and stores an amount of correction oflight quantity falloff at edges (hereinafter referred to as “profiledata”) for each block. During one scanning operation, the system readsprofile data of a target block and profile data of a neighboring block,and controls a driving current value of the laser using linearlyinterpolated profile data obtained from the read data to correct lightquantity falloff at edges.

FIG. 12 illustrates a conventional correction circuit for correctinglight quantity falloff at edges. The correction circuit includes adigital/analog (D/A) converter 602 and a low-pass filter 604.

The D/A converter 602 receives a sampled-and-held voltage value VSH(i.e., a voltage value corresponding to a maximum light quantity) as areference voltage to perform APC processing in a light detection period(i.e., the BD period) during one scanning operation. The D/A converter602 also receives light quantity correction data 203 from a lightquantity correction unit (not illustrated) and a clock signal CLK. TheD/A converter 602 performs digital/analog conversion processing on thevoltage value VSH based on the light quantity correction data 203 andthe clock signal CLK.

The low-pass filter 604 is, for example, composed of a capacitor and aresistor. The low-pass filter 604 can filter an analog signal S605received from the D/A converter 602. The filter 604 outputs an analogsignal VCOM (i.e., a filtered signal) that can be used to control thecurrent of a pulse current source of a laser drive control unit (notillustrated). Thus, the correction circuit can variably control thequantity of light during one scanning operation and can correct lightquantity falloff at edges.

However, the following problem arises when the above-describedconventional circuit corrects the driving current of a laser accordingto an amount of correction of light quantity falloff at edges of anoptical system to solve the unevenness of density of an image.

More specifically, even if the rate of light quantity falloff at edgesis 10% or 20%, i.e., even if the light quantity deteriorates from 100%to 90% or 80% during one scanning operation, a dynamic range for the D/Aconversion is set to the light quantity of 100% to 0%.

For example, the above-described dynamic range setting is required torealize the correction of light quantity falloff at edges for a lowlight quantity of approximately 50%. As 1LSB (i.e., a minimum resolutionof D/A) is a value determined in relation to the 100% light quantity,the resolution deteriorates according to a decrease in the lightquantity. To solve this problem, the D/A of a higher resolution isrequired. Furthermore, re-calculation of correction data is required ifthere is any change in the light quantity to be used. Thus, the controlbecomes complicated.

The above-described problem is described in more detail with referenceto FIGS. 11A and 11B. FIG. 11A illustrates a light quantity distributionon a drum surface including a BD image height (i.e., an image height ata BD position) at a light quantity level of P₀. In FIG. 11A. P_(0BD)represents a light quantity at the BD image height, P_(X0) represents alight quantity at an image height X₀, . . . , and P_(XN) represents alight quantity at an image height X_(N). Furthermore,(P₀-P_(X0))/P_(X0), . . . , and (P₀-P_(XN))/P_(XN) represent correctionamounts Y₀, . . . , and Y_(N) at respective image heights X₀ . . .X_(N).

Namely, each of the correction amounts Y₀, . . . , and Y_(N) is a ratioof a difference between a target light quantity and an actual lightquantity to the actual light quantity at each image height. The lightquantity correction is performed by adding the correction amount to thelaser driving current according to the image height, to have the targetlight quantity at each image height, as indicated by a corrected profileP′₀ in FIG. 11A. In this case, if a D/A converter has a resolution of 8bits, the minimum resolution 1LSB becomes P₀/255. If the rate of lightquantity falloff at edges at the light quantity level P₀ is 20%, thecorrection data is somewhere in a range of 0 to 51(=255×20%) at eachimage height.

FIG. 11B illustrates a comparative light quantity distribution on a drumsurface at a light quantity level of 0.5P₀ (i.e., a half of the lightquantity P₀). In this case, the rate of light quantity falloff at edgesis reduced to a half value. As illustrated in FIG. 11B, correctionamounts Y′₀, . . . , and Y′_(N) corresponding to the light quantitylevel of 0.5P₀ are (0.5P₀-0.5P_(X0))/P_(X0)=0.5(P₀-P_(X0))/P_(X0)=0.5Y₀,. . . , and (0.5P₀-0.5P_(XN))/P_(XN)=0.5 (P₀-P_(XN))/P_(XN)=0.5Y_(N) atrespective image heights.

Namely, if the light quantity in the acquisition of profile data isdifferent from the actually used light quantity, the same profile datacannot be used. For example, if the rate of light quantity falloff atedges is 20% and an 8-bit D/A converter is used at the light quantitylevel of 0.5P₀, the correction data is somewhere in a range of 0 to 26(=255×20% x50%) at each image height. The resolution deteriorateslargely compared to the correction range from 0 to 51, even if thecorrection rate for the light quantity is the same (20%). The resolutionfurther deteriorates according to a decrease in the light quantity.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention are directed to anoptical scanning apparatus capable of correcting light quantity falloffat edges at a higher correction resolution. The optical scanningapparatus can maintain a sufficient correction resolution even if thereis any change in the light quantity, and does not require are-calculation of correction data.

According to an aspect of the present invention, an optical scanningapparatus capable of controlling an output of a light source for formingan electrostatic latent image on an image carrier includes a correctionamount control unit configured to variably control a light quantitycorrection amount of the light source according to a scanning positionon the image carrier during one scanning operation with a beam generatedfrom the light source, an output signal level changing unit configuredto change a level of an output signal from the correction amount controlunit, and a light quantity control unit configured to control alightquantity of the light source according to the scanning position based ona signal from the output signal level changing unit.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain at least some of the principles of the invention.

FIG. 1 is a cross-sectional diagram illustrating an image formingapparatus according to an exemplary embodiment of the present invention.

FIG. 2 illustrates an exposure control unit illustrated in FIG. 1according to an exemplary embodiment of the present invention.

FIG. 3 illustrates a correction unit according to an exemplaryembodiment of the present invention.

FIGS. 4A to 4C illustrate correction processing performed by thecorrection unit of the exposure control unit according to an exemplaryembodiment of the present invention.

FIG. 5 illustrates correction data stored in a memory of the correctionunit according to an exemplary embodiment of the present invention.

FIG. 6 illustrates generation of linearly interpolated data by thecorrection unit according to an exemplary embodiment of the presentinvention.

FIG. 7 illustrates a laser drive control unit according to an exemplaryembodiment of the present invention.

FIG. 8 illustrates an APC circuit in the laser drive control unitaccording to an exemplary embodiment of the present invention.

FIG. 9 illustrates a pulse current change control unit in the laserdrive control unit according to an exemplary embodiment of the presentinvention.

FIGS. 10A to 10E illustrate an operation of the pulse current changecontrol unit in the laser drive control unit according to an exemplaryembodiment of the present invention.

FIG. 11A illustrates a profile in a conventional example in which thelight quantity of a laser is not changed.

FIG. 11B illustrates a profile in a conventional example in which thelight quantity of a laser is changed.

FIG. 12 illustrates an example of a conventional correction circuit forcorrecting light quantity falloff at edges.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following description of exemplary embodiments is illustrative innature and is in no way intended to limit the invention, itsapplication, or uses. It is noted that throughout the specification,similar reference numerals and letters refer to similar items in thefollowing figures, and thus once an item is described in one figure, itmay not be discussed for following figures. Exemplary embodiments willbe described in detail below with reference to the drawings.

FIG. 1 is a vertical cross-sectional diagram illustrating an imageforming apparatus according to an exemplary embodiment of the presentinvention.

The image forming apparatus includes a document feeder 1 thatsuccessively conveys document sheets onto a document positioning glassplate 2. When a document sheet reaches a predetermined position on thedocument positioning glass plate 2, a lamp 3 turns on and a scanner unit4 moves in a horizontal direction so that the entire surface of the laiddocument can be illuminated by the lamp 3. The reflection light from thedocument sheet sequentially reflects on mirrors 5, 6, and 7 and passesthrough a lens 8, and finally forms an optical image on an imagingsurface of an image sensor unit 9.

The image sensor unit 9, having a photoelectric conversion function,converts the optical image into an electric signal. An image processingunit (not illustrated) receives the electric signal. The imageprocessing unit converts the input electric signal into a digital signaland applies image processing to the digital signal. An image signalgenerated from the image processing unit is directly, or aftertemporarily stored in an image memory, sent to an exposure control unit10 that functions as an optical scanning apparatus.

The exposure control unit 10 controls a light source, i.e., asemiconductor laser 43 (illustrated in FIG. 2), according to an inputimage signal. The semiconductor laser 43 emits a controlled laser beam,which is transmitted via a scanning system including a polygonal mirror33 (see FIG. 2) onto a photosensitive drum 11 that functions as a rotaryimage carrier. The laser beam repetitively scans in the main scanningdirection on the photosensitive drum 11.

Thus, an electrostatic latent image corresponding to the image signalcan be formed on the photosensitive drum 11. An auxiliary chargingdevice 26, a pre-exposure lamp 27, a primary charging device 28, apotential sensor 100, a developing device 13, a transfer device 16, anda cleaner 25 are disposed around the photosensitive drum 11.

The auxiliary charging device 26 has a function of preliminarilyeliminating electrostatic charge on the surface of the photosensitivedrum 11. The pre-exposure lamp 27 can remove residual charge from thesurface of the photosensitive drum 11. The primary charging device 28can uniformly charge the surface of the photosensitive drum 11. Thepotential sensor 100 measures an electric potential on the surface ofthe photosensitive drum 11. The potential sensor 100 is, for example,composed of six sensors positioned at equal intervals in the mainscanning direction (i.e., in the axial direction of the photosensitivedrum 11).

The developing device 13 supplies toner onto the surface of thephotosensitive drum 11, so that an electrostatic latent image on thephotosensitive drum 11 can be visualized as a toner image. The transferdevice 16 transfers the toner image from the photosensitive drum 11 to asheet having been fed from a cassette 14 or 15. The cleaner 25 removesresidual toner from the photosensitive drum 11 and cleans the surface ofthe photosensitive drum. 11 in preparation for the next image formingoperation

A sheet with a toner image transferred by the transfer device 16 isconveyed to a fixing unit 17. The fixing unit 17 thermally presses andfixes the toner image on the sheet. The sheet with the fixed toner imageis guided by a flapper 20 toward discharge rollers 18, which dischargethe sheet to the outside of the image forming apparatus.

In a two-sided print mode, after an image is formed on one surface of asheet, the flapper 20 performs a switching operation for reversing theimage forming surface to the other surface. According to this sheetreversing operation, the sheet is guided toward a two-sided path 24.Then, the sheet is conveyed from the two-sided path 24 to a spacebetween the photosensitive drum 11 and the transfer device 16. Then, atoner image is transferred to the other surface of the sheet.

FIG. 2 illustrates an exemplary configuration of the exposure controlunit 10. The exposure control unit 10 includes a laser drive controlunit 54, which causes the semiconductor laser 43 to emit a laser beam.The semiconductor laser 43 includes a built-in photo diode (PD) sensor43B (see FIG. 7) that can detect part of the emitted laser beam. Adetection signal from the PD sensor 43B can be used for automatic powercontrol (APC) of the semiconductor laser 43.

A collimator lens 35 and a diaphragm 32, associated together in apredetermined positional relationship, can convert a laser beam from thesemiconductor laser 43 into a parallel beam that has a predetermineddiameter, which reaches the polygonal mirror 33. The polygonal mirror 33rotates at a constant angular speed in a direction indicated by an arrowin FIG. 2. In other words, the rotating polygonal mirror 33 can convertan incident laser beam into a deflecting beam that continuously changesthe angle on a horizontal plane. The deflecting beam is subjected to alight collection function of an f-θ lens 34. The f-θ lens 34 correctsdistortion of a laser beam to assure temporal scanning linearity. Thus,the laser beam can scan and form a latent image on the photosensitivedrum 11 at a constant speed.

A beam detection (BD) sensor 36, which can detect a laser beam reflectedfrom the polygonal mirror 33 and passing through the f-θ lens 34, isprovided near an axial edge of the photosensitive drum 11. A detectionsignal 201 from the BD sensor 36 can be used as a synchronization signalfor synchronizing the rotation of the polygonal mirror 33 with a writingoperation of data.

An input unit 61 outputs light quantity correction data 72 to acorrection unit 52. The input unit 61 is, for example, a bar-code readerthat can read information from a bar code or an operation unit thatenables a user to input various data. Alternatively, the input unit 61can be any other device capable of inputting correction data, such as anelectronically erasable and programmable read-only memory (EEPROM) thatstores correction data for light quantity falloff at edges.

The correction unit 52 generates a BD mask signal 301 in synchronismwith the BD signal 201 from the BD sensor 36 and outputs the generatedBD mask signal 301 to the laser drive control unit 54. Furthermore, thecorrection unit 52 outputs the light quantity correction data 203 to thelaser drive control unit 54 according to a scanning position of thelaser beam on the photosensitive drum 11.

The laser drive control unit 54 has a D/A conversion function forconverting the light quantity correction data 203 received from thecorrection unit 52 into an analog signal. During an image forming periodfor forming a latent image on the photosensitive drum 11, the laserdrive control unit 54 controls a current value and a drive time of adriving (light emission) signal 204 for the semiconductor laser 43 basedon an image signal 202 received from an image signal generation unit 53and the light quantity correction data 203.

In this manner, the laser beam emitted from the semiconductor laser 43becomes a parallel beam having a predetermined diameter via thecollimator lens 35 and the diaphragm 32 and falls on the polygonalmirror 33.

FIG. 3 is an exemplary circuit diagram of the correction unit 52according to an exemplary embodiment of the present invention.

The correction unit 52 includes a memory 74, a counter 75, a latch unit76, an arithmetic processing unit 77, a comparator 505, a comparator506, and a JK flip-flop 507.

FIG. 5 illustrates correction data stored in the memory 74. The counter75 is reset in response to the BD signal 201 and counts a pixel clocksignal (CLK) synchronized with pixel data to be recorded. The counter 75is, for example, a 4-bit counter, which generates an output (Q) 401,i.e., a carry signal, generated every time the counter 75 counts up to“16.” The count number “16” of the counter 75 corresponds to the numberof pixels in one correction block (i.e., a correction block between twoneighboring dotted lines in FIG. 4) of each scanning line illustrated inFIG. 4, and can be used for reading correction data of each correctionblock (sample point).

The memory 74 stores the light quantity correction data 72 for theexposure control unit 10, which is received from the input unit 61 via abus 509. The memory 74 is, for example, a first-in first-out (FIFO)memory and outputs correction data X of each correction block insynchronism with the output (Q) 401 of the counter 75 (i.e., every timethe counter 75 counts up to “16”). Namely, as illustrated in FIG. 5, thememory 74 successively outputs the correction data X from address “0” toaddress “n” in the main scanning direction.

The latch unit 76 latches the correction data X received from the memory74 in synchronism with the output (Q) 401 of the counter 75. Thearithmetic processing unit 77 receives two input signals ADATA andBDATA, i.e., an output (W) of the latch unit 76 and an output (X) of thememory 74. Based on the two input signals ADATA and BDATA, thearithmetic processing unit 77 generates linearly interpolated data 203,which is output to the laser drive control unit 54. Namely, thearithmetic processing unit 77 performs a linear interpolation using theNth correction data (BDATA) and the (N-1)th correction data (ADATA) inthe main scanning direction.

Furthermore, the counter 75 outputs a count output 510 (i.e., a countvalue) to the comparator 505 and the comparator 506. The comparator 505receives another input 501 representing a position (CLK number) from theBD signal 201 to a rise of the BD mask signal 301, which can be set by acentral processing unit (CPU) (not illustrated) or a register (notillustrated). Similarly, the comparator 506 receives another input 502representing a position (CLK number) from the BD signal 201 to a fall ofthe BD mask signal 301, which can be set by a CPU (not illustrated) or aregister (not illustrated).

Referring to the count output 510 of the counter 75, the comparator 505outputs a signal 503 having a high level at a rise position of the BDmask signal 301 and otherwise a low level. Similarly, the comparator 506outputs a signal 504 having a high level at a fall position of the BDmask signal 301 and otherwise a low level. The comparator signal 503 istransmitted to a J terminal of the JK flip-flop 507. The comparatorsignal 504 is transmitted to a K terminal of the JK flip-flop 507. TheJK flip-flop 507 is reset to a low level in response to the BD signal201. The JK flip-flop 507 outputs the BD mask signal 301 to the laserdrive control unit 54 when both the comparator signals 503 and 504 areat a high level.

An exemplary method for correcting light quantity falloff at edges bythe above-described optical scanning apparatus is described below withreference to FIGS. 4A to 4C.

FIG. 4A illustrates optical characteristics 70 of the exposure controlunit 10, which indicates a rate of light quantity falloff at edges ineach block when the image height is divided into a plurality of blocks(sample points). According to the optical characteristics of FIG. 4A,the maximum rate of light quantity falloff at edges is 20%. In FIG. 4A,the ordinate axis represents the rate of light quantity falloff at edgesand the abscissa axis represents the image height.

FIG. 4B illustrates an example of the correction data 72, whichindicates correction data in each block for correcting the opticalcharacteristics 70 illustrated in FIG. 4A. In FIG. 4B, FFH represents acorrection level for a block where the rate of light quantity falloff atedges is maximum in the optical characteristics 70 (i.e., a block havinga rate of light quantity falloff at edges of 20%, which is the n-thblock in the main scanning direction), and OOH represents a correctionlevel for a block where the rate of light quantity falloff at edges isminimum (i.e., a block having a rate of light quantity falloff at edgesof 0%). FIG. 4B indicates correction data for light quantity falloff atedges in each block of the image height, which can be input from theinput unit 61 to the correction unit 52.

FIG. 4C illustrates correction data 203 obtained by linearlyinterpolating the correction data 72 of two neighboring blocks, i.e.,Nth correction data and (N-1)th correction data in the main scanningdirection.

The correction unit 52 generates the BD mask signal 301 having a lowlevel before and after the BD signal 201 is detected and otherwisehaving a high level. The correction unit 52 transmits theabove-described linearly interpolated correction data 203 and the BDmask signal 301 to the laser drive control unit 54.

FIG. 5 illustrates correction data stored in the memory 74 of thecorrection unit 52. The memory 74 receives the correction data 72 fromthe input unit 61 via the bus 509 and stores the received correctiondata 72 as 0th to n-th correction data in order of address in the mainscanning direction (correction data for a plurality of sample points).Namely, each correction data in the memory 74 is stored in relation toan allocated address. The correction unit 52 resets the counter 75(i.e., internal address counter) in response to the BD signal 201, andsuccessively outputs correction data from the address “0” to the address“n” in response to the CLK signal.

According to an exemplary embodiment, the memory 74 stores correctiondata received from the input unit 61. However, if the input unit 61includes an EEPROM capable of functioning as the memory 74, thecorrection data can be directly read out of the EEPROM.

FIG. 6 illustrates generation of the linearly interpolated correctiondata 203 by the correction unit 52 illustrated in FIG. 3. In the presentembodiment, one scanning line in the main scanning direction correspondsto 2048 pixels. One scanning line is divided into a total of 128 (n=127)correction blocks (H1 to H128). Each correction block corresponds to 16pixels (m=16) (2048÷127=16).

The latch unit 76 illustrated in FIG. 3 generates an output W. The latchunit 76 latches data X output from the memory 74 in synchronism with theoutput (Q) 401 of the counter 75. The output (Q) 401 is a carry signaloutput from the counter 75 each time the counter 75 counts up to “16.”

The memory 74 successively outputs the correction data X of eachcorrection block in synchronism with the output (Q) 401 of the counter75 (each time the counter 75 counts up to “16”), from the 0th correctiondata to the n-th correction data in the main scanning direction, asillustrated in FIG. 5.

The arithmetic processing unit 77 calculates the linearly interpolatedcorrection data 203 based on two input signals ADATA and BDATA (i.e.,the output (W) of the latch unit 76 and the output (X) of the memory 74)and outputs the linearly interpolated correction data 203 to the laserdrive control unit 54. Namely, the arithmetic processing unit 77performs a linear interpolation on the Nth correction data (BDATA) andthe (N-1) th correction data (ADATA) and generates the linearlyinterpolated correction data 203.

Next, exemplary linear interpolation processing performed by thecorrection unit 52 is described below with reference to FIGS. 3, 5, and6. Each correction block of one scanning line corresponds to 16 pixels(m=16). One scanning line corresponds to 2048 pixels. The number ofpixels within a correction interval on one scanning line is “128” (n=127in FIG. 5).

When the memory 74 receives the BD signal 201 from the BD sensor 36 viathe reset terminal (RST), the memory 74 clears the address to “0.”Similarly, when the counter 75 receives the BD signal 201 from the BDsensor 36 via the reset terminal (RST), the counter 75 clears the countvalue to “0.” The counter 75 is a 4-bit counter. The output 401 of thecounter 75 is a carry signal.

First, if a dummy clock signal (CLK) of 16 clocks is received for acorrection block H1, the counter 75 outputs the output 401, which is apulse signal of one clock. The latch unit 76 latches the output of thememory 74 (i.e., 0th correction data in the main scanning direction).The output of the memory 74 becomes first correction data in the mainscanning direction, which is stored in the next address. Thus, thearithmetic processing unit 77 can receive the input ADATA (i.e., the 0thcorrection data (W) in the main scanning direction) from the latch unit76 and the input BDATA (i.e., the first correction data (X) in the mainscanning direction) from the memory 74.

The arithmetic processing unit 77 obtains a difference between twoinputs W and X. The arithmetic processing unit 77 linearly interpolatesthe obtained difference with “16” corresponding to the number of pixelsof one correction block, and outputs the linearly interpolatedcorrection data 203 in synchronism with the pixel clock signal (CLK).

Namely, the arithmetic processing unit 77 performs linear interpolationprocessing on two correction data stored in neighboring addresses of thememory 74. The arithmetic processing unit 77 outputs new correction data203 representing linearly interpolated correction data (1 to m in FIG.6, where m=16).

After the correction processing for the correction block H1 is completedin this manner, the counter 75 outputs the pulse signal 401 to initiatecorrection processing for the next correction block H2. The latch unit76 latches the output (W) of the memory 74 (i.e., first correction datain the main scanning direction). The output of the memory 74 becomessecond correction data (X) in the main scanning direction, which isstored in the next address.

The arithmetic processing unit 77 obtains a difference between twoinputs W and X. The arithmetic processing unit 77 linearly interpolatesthe obtained difference with “16” corresponding to the number of pixelswithin one correction interval, and outputs the linearly interpolatedcorrection data 203 in synchronism with the pixel clock signal (CLK).Similarly, the arithmetic processing unit 77 outputs the linearlyinterpolated correction data 203 for each correction block (H3 to H128).

The memory 74 is an FIFO memory according to the example illustrated inFIG. 3. However, the memory 74 can be replaced with another type ofmemory. For example, the memory 74 can be an ordinary memory (RAM),which can receive a count value of the counter 75 as an address of thememory 74. In this case, according to the above-described example, thememory can include a 4-bit counter functioning as a first-stage counterand a second-stage counter that counts a carry output of the 4-bitcounter. An output of the second-stage counter can be used as an addressof the memory 74.

Furthermore, according to the above-described example, the memory 74 hasa memory space of 1028 addresses. The 0th correction data in the mainscanning direction is stored in a memory space ranging from address 0 toaddress 15 in the memory 74. The first correction data in the mainscanning direction is stored in a memory space ranging from address 16to address 31 in the memory 74. The second correction data in the mainscanning direction is stored in a memory space ranging from address 32to address 47 in the memory 74. One counter and the memory 74 canrealize an operation similar to the operation realized by the counter 75and the memory 74 illustrated in FIG. 3.

FIG. 7 illustrates an example of the laser drive control unit 54 in theexposure control unit 10 according to an exemplary embodiment.

The semiconductor laser 43 includes a laser diode 43A and a PD sensor43B. A bias current source 41 supplies bias current to the laser diode43A. A pulse current source 42 supplies pulse current to the laser diode43A. A modulation unit 48 performs pixel modulation processing on theimage signal 202 received from the image signal generation unit 53. Aswitch 49 turns on or off according to a signal from a logic element 40that obtains a logical sum of the modulation signal and a full-lightingsignal FULL (i.e., a BD detection signal) supplied from a sequencecontroller 47.

When the switch 49 is in an ON state, the laser diode 43A emits lighthaving an intensity corresponding to a sum of the current supplied fromthe bias current source 41 and the current supplied from the pulsecurrent source 42. The current supplied from the bias current source 41is stationary during one scanning operation. The current supplied fromthe pulse current source 42 is variable during one scanning operation.When the switch 49 is in an OFF state, the laser diode 43A emits lighthaving an intensity corresponding to only the current supplied from thebias current source 41.

The PD sensor 43B monitors the quantity of light during a full-lightingoperation for the BD detection (i.e., when the full-lighting signal FULLis in an active stare). A current/voltage (I/V) converter 44 converts anoutput signal from the PD sensor 43B into a voltage signal. An amplifier45 amplifies an output signal from the converter 44. An APC circuit 46receives a signal VPD representing an output signal from the amplifier45.

The APC circuit 46 samples the signal VPD in response to asample-and-hold signal supplied from the sequence controller 47 during aBD detection period. The APC circuit 46 holds the sampled signal duringone scanning operation and outputs the sampled-and-held signal as a VSHsignal to a pulse current change control unit 50. At the same time, theAPC circuit 46 compares the VSH signal with a predetermined voltagevalue corresponding to a maximum light quantity and generates adifference signal VAPC to control the bias current of the bias currentsource 41.

The pulse current change control unit 50 receives the VSH signal fromthe APC circuit 46, and also receives correction data, a target lightquantity setting value, and a maximum rate of light quantity falloff atedges from the sequence controller 47. The pulse current change controlunit 50 outputs a signal VCOM to control the pulse current of the pulsecurrent source 42.

FIG. 8 is a circuit diagram illustrating an example of the APC circuit46 according to an exemplary embodiment. The APC circuit 46 includes ananalog switch 38 that samples an amplified PD sensor output VPD inresponse to a sample-and-hold signal S/H supplied from the sequencecontroller 47. The analog switch 38 holds a sampled voltage value VSHduring one scanning operation at a time constant determined by aresistor 37 and a capacitor 39.

A comparator 40 compares the voltage value VSH with a predeterminedvalue VREF. The comparator 40 outputs the difference signal VAPC tocontrol the current of the bias current source 41. Namely, the APCcircuit 46 controls the current of the bias current source 41 in eachscanning operation such that a bias light-emission value becomes equalto a target value being set by the predetermined value VREF. Thus, theAPC circuit 46 can equalize the bias light quantity of the laser diode43A with a desired light quantity.

The image forming apparatus according to an exemplary embodimentperforms light quantity correction during one scanning operation, inaddition to the above-described APC processing. The pulse current changecontrol unit 50 generates a signal VCOM to control a pulse currentamount of the pulse current source 42 during one scanning operation.

FIG. 9 is a circuit diagram illustrating an exemplary configuration ofthe pulse current change control unit 50 according to an exemplaryembodiment.

The pulse current change control unit 50 includes a multiplier 601(e.g., an operational amplifier). The multiplier 601 multiples thevoltage value VSH (sampled-and-held voltage signal) received from theAPC circuit 46 by a target light quantity setting value S601 (i.e., avalue equal to or less than “1”) supplied from a CPU (not illustrated),and generates an output voltage S602 representing a multiplied value.The pulse current change control unit 50 includes a D/A converter 602that receives the output voltage S602 from the multiplier 601 as areference voltage. The D/A converter 602 performs D/A conversionprocessing on the output voltage S602 based on the linearly interpolatedcorrection data 203 supplied from the correction unit 52 and the clocksignal CLK and generates an output voltage S605.

Furthermore, the pulse current change control unit 50 includes amultiplier 603 (e.g., an operational amplifier). The multiplier 603multiplies the output voltage S605 supplied from the D/A converter 602by a maximum rate of light quantity falloff at edges S606 supplied fromthe sequence controller 47, and generates an output voltage S607representing a multiplied value.

According to an exemplary embodiment, the maximum rate of light quantityfalloff at edges S606 is 20%. Thus, the constant multiplied by themultiplier 603 is 0.2 (i.e., a value smaller than 21″ and corresponds toa maximum rate for realizing the light quantity correction). A low-passfilter 604 is, for example, composed of a capacitor and a resistor. Anadder 605 is an operational amplifier that generates an output voltageS608 representing an addition of the output voltage of the low-passfilter 604 and the output voltage S602 of the multiplier 601. An analogswitch 606 selectively outputs the voltage value VSH or the outputvoltage S608 (i.e., a signal filtered by the low-pass filter 604 andoffset by the adder 605) according to the BD mask signal 301. If themaximum rate of light quantity falloff at edges S606 is 30%, theconstant multiplied by the multiplier 603 is 0.3. If the maximum rate oflight quantity falloff at edges S606 is 50%, the constant multiplied bythe multiplier 603 is 0.5. A VCOM signal is output from the analogswitch 606.

FIGS. 10A to 10E illustrate an exemplary flow of signals in the pulsecurrent change control unit 50 illustrated in FIG. 9. FIG. 10Aillustrates linear interpolation correction data corresponding to eachblock image height illustrated in FIG. 4C. FIG. 10B illustrates anexemplary case where the target light quantity setting value S601 is “1”(e.g., the target light quantity is 100%).

In FIG. 10B, as the target light quantity setting value S601 is “1”, theoutput voltage S602 of the multiplier 601 is equal to thesampled-and-held voltage VSH obtained in the APC processing. The D/Aconverter 602 receives the sampled-and-held voltage VSH (i.e., thereference voltage for the D/A conversion) from the multiplier 601 andalso receives the linearly interpolated correction data 203. The D/Aconverter 602 performs the D/A conversion processing on the input datasuch that the sampled-and-held voltage VSH can be output when thecorrection data 203 is FFH.

The multiplier 603 receives the analog converted voltage S605 from theD/A converter 602 and the maximum rate of light quantity falloff atedges S606 (i.e., 20%=0.2 in this example) from the sequence controller47. The multiplier 603 multiples the voltage S605 by 0.2 and generatesthe output signal S607. The adder 605 receives the output signal S607(=0.2×S605) via the low-pass filter 604 and generates the voltage S608,which is offset by an amount equivalent to the voltage VSH. The analogswitch 606 receives the output voltage S608 from the adder 605.

The analog switch 606 selects an output voltage (i.e., a light quantitysignal for obtaining a main scanning synchronizing signal in eachscanning operation) according to the BD mask signal 301. Morespecifically, the analog switch 606 outputs the voltage S608 in an imageregion such that an ordinary APC operation can be carried out during aBD period. The analog switch 606 outputs the voltage VSH in a non-imageregion. The pulse current source 42 is controlled according to a VCOMsignal output from the analog switch 606.

FIG. 10C illustrates the quantity of light emitted from the laser 43according to the current of the pulse current source 42. The lightquantity on the drum surface becomes a flat light quantity of P0.Namely, the pulse current change control unit 50 applies the D/Aconversion to a rate of light quantity falloff at edges of 20% in afull-range. Thus, the correction resolution (1LSB of D/A) is 0.078%(=20%/256), which is excellent compared to 0.39% (=100%/256) accordingto a conventional example.

FIG. 10D illustrates an exemplary case where the target light quantityis reduced to a half level. In this case, the target light quantitysetting value S601 received from the sequence controller 47 is 0.5.Thus, the multiplier 601 generates the output voltage S602 (=0.5×VSH) asillustrated in FIG. 10D. The D/A converter 602 receives the voltage S602(i.e., the reference voltage) from the multiplier 601 and also receivesthe linearly interpolated correction data 203.

The D/A converter 602 performs the D/A conversion processing on theinput data such that the voltage 0.5VSH can be output when thecorrection data 203 is FFH. The multiplier 603 receives the analogconverted voltage S605 from the D/A converter 602 and the maximum rateof light quantity falloff at edges S606 (i.e., 20%=0.2 in this example)from the sequence controller 47. The multiplier 603 multiples thevoltage S605 by 0.2 and generates the output signal S607.

The adder 605 receives the output signal S607 (=0.2×S605) via thelow-pass filter 604 and generates the voltage S608, which is offset byan amount equivalent to the voltage 0.5VSH. The analog switch 606receives the voltage S608 from the adder 605. The analog switch 606selects an output voltage according to the BD mask signal 301. Morespecifically, the analog switch 606 outputs the voltage S608 in an imageregion such that an ordinary APC operation can be carried out during aBD period. The analog switch 606 outputs the voltage VSH in a non-imageregion. The pulse current source 42 is controlled according to the VCOMsignal output from the analog switch 606.

FIG. 10E illustrates the quantity of light emitted from the laser 43according to the current of the pulse current source 42. The lightquantity during a BD period is P0. The light quantity on the drumsurface becomes a flat light quantity of P0/2. Namely, the lightquantity during a BD period is unchanged even if the target lightquantity is reduced to a half value. The BD synchronization accuracydoes not deteriorate. The light quantity correction data can be usedwithout any change. Therefore, it is unnecessary to re-calculate thecorrection data every time the target light quantity is changed.

Namely, the pulse current change control unit 50 applies the D/Aconversion to a maximum rate of light quantity falloff at edges of 20%in a full-range of 00H to FFH. Thus, the correction resolution (1LSB ofD/A) is 0.078% (=20%/256), which is excellent compared to 0.39%(=100%/256) according to a conventional example.

As described above, an exemplary embodiment can improve the correctionresolution by multiplying a maximum rate of light quantity falloff atedges by the light quantity correction data subjected to the D/Aconversion processing. Thus, light quantity correction data of afull-range (corresponding to a maximum rate of light quantity falloff atedges) can be input to the D/A converter.

Furthermore, an exemplary embodiment is configured to switch between alight quantity in a BD detection operation and a target light quantityfor correcting light quantity falloff at edges, so that the D/Areference voltage can be changed according to the target light quantity.Thus, the BD synchronization accuracy does not deteriorate. An exemplaryembodiment does not require a re-calculation of light correction dataeven if the target light quantity is changed. Thus, the load of a CPUcan be reduced.

Furthermore, an exemplary embodiment corrects light quantity falloff atedges (including only one peak in the light quantity distribution) inthe optical characteristics of an optical scanning apparatus. However,if the sensitivity of a drum has a plurality of peaks depending on theimage height, a similar control operation can be performed to correctthe unevenness although different light quantity correction data isused.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all modifications, equivalent structures, and functions.

1. An optical scanning apparatus for controlling an output of a lightsource for forming an electrostatic latent image on an image carrier,the optical scanning apparatus comprising: a correction amount controlunit configured to variably control a light quantity correction amountof the light source according to a scanning position on the imagecarrier during one scanning operation with a beam generated from thelight source; an output signal level changing unit configured to changea level of an output signal from the correction amount control unit; anda light quantity control unit configured to control a light quantity ofthe light source according to the scanning position based on a signalfrom the output signal level changing unit.